Anthracyclines are a class of antibiotics derived from certain types of Streptomyces bacteria. Anthracyclines are often used as cancer therapeutics and function in part as nucleic acid intercalating agents and inhibitors of the DNA repair enzyme topoisomerase II, thereby damaging nucleic acids in cancer cells, preventing the cells from replicating. One example of an anthracycline cancer therapeutic is doxorubicin, which is used to treat a variety of cancers including breast cancer, lung cancer, ovarian cancer, lymphoma, and leukemia. The 6-maleimidocaproyl hydrazone of doxorubicin (DOXO-EMCH) was originally synthesized to provide an acid-sensitive linker that could be used to prepare immunoconjugates of doxorubicin and monoclonal antibodies directed against tumor antigens (Willner et al., Bioconjugate Chem 4:521-527 (1993)). In this context, antibody disulfide bonds are reduced with dithiothreitol to form free thiol groups, which in turn react with the maleimide group of DOXO-EMCH to form a stable thioether bond. When administered, the doxorubicin-antibody conjugate is targeted to tumors containing the antigen recognized by the antibody. Following antigen-antibody binding, the conjugate is internalized within the tumor cell and transported to lysosomes. In the acidic lysosomal environment, doxorubicin is released from the conjugate intracellularly by hydrolysis of the acid-sensitive hydrazone linker. Upon release, the doxorubicin reaches the cell nucleus and is able to kill the tumor cell. For additional description of doxorubicin and DOXO-EMCH see, for example, U.S. Pat. Nos. 7,387,771 and 7,902,144 and U.S. patent application Ser. No. 12/619,161, each of which are incorporated in their entirety herein by reference.
A subsequent use of DOXO-EMCH was developed by reacting the molecule in vitro with the free thiol group (Cys-34) on human serum albumin (HSA) to form a stable thioether conjugate with this circulating protein (Kratz et al., J Med Chem 45:5523-5533 (2002)). Based on these results, it was hypothesized that intravenously-administered DOXO-EMCH would rapidly conjugate to HSA in vivo and that this macromolecular conjugate would preferentially accumulate in tumors due to an “enhanced permeability and retention” (EPR) intratumor effect (Maeda et al., J Control Release 65:271-284 (2000)).
Acute and repeat-dose toxicology studies with DOXO-EMCH in mice, rats, and dogs identified no toxicity beyond that associated with doxorubicin, and showed that all three species had significantly higher tolerance for DOXO-EMCH compared to doxorubicin (Kratz et al., Hum Exp Toxicol 26:19-35 (2007)). Based on the favorable toxicology profile and positive results from animal tumor models, a Phase 1 clinical trial of DOXO-EMCH was conducted in 41 advanced cancer patients (Unger et al., Clin Cancer Res 13:4858-4866 (2007)). This trial found DOXO-EMCH to be safe for clinical use. In some cases, DOXO-EMCH induced tumor regression.
Due to the sensitivity of the acid-cleavable linker in DOXO-EMCH, it is desirable to have formulations that are stable in long-term storage and during reconstitution (of, e.g., previously lyophilized compositions) and administration. DOXO-EMCH, when present in compositions, diluents and administration fluids used in current formulations, is stable only when kept at low temperatures. The need to maintain DOXO-EMCH at such temperatures presents a major problem in that it forces physicians to administer cold (4° C.) DOXO-EMCH compositions to patients. Maintaining DOXO-EMCH at low temperatures complicates its administration in that it requires DOXO-EMCH to be kept at 4° C. and diluted at 4° C. to prevent degradation that would render it unsuitable for patient use. Further, administration at 4° C. can be harmful to patients whose body temperature is significantly higher (37° C.).
Lyophilization has been used to provide a stable formulation for many drugs. However, reconstitution of lyophilized DOXO-EMCH in a liquid that does not maintain stability at room temperature can result in rapid decomposition of DOXO-EMCH. Use of an inappropriate diluent to produce an injectable composition of DOXO-EMCH can lead to decreased stability and/or solubility. This decreased stability manifests itself in the cleavage of the linker between the doxorubicin and EMCH moieties, resulting in degradation of the DOXO-EMCH into two components: doxorubicin and linker-maleimide. Thus, stable, reconstituted lyophilized solutions of anthracycline-EMCH (e.g., DOXO-EMCH), and injectable compositions containing the same, are required to solve these problems and to provide a suitable administration vehicle that can be used reasonably in treating patients both for clinical trials and commercially.